1. Field of the Invention
The present invention relates to profiles of pulley sheaves for the variable pulleys of a continuously variable transmission. More specifically, this invention relates to a variable pulley sheave profile and a method for generating the same providing a substantially error-free alignment at all drive ratios.
2. Description of the Prior Art
In recent years, significant research and development has been devoted to a practical continuously variable transmission (CVT) for automotive applications. A CVT provides a portion of the mechanical link between the vehicle engine and the drive wheels used to control the torque output of the engine.
A CVT generally operates by the use of multiple variable pulleys mounted on parallel axes, connected by an endless chain-belt of metal or elastomeric material. A first variable pulley is situated on an input shaft and is mechanically driven by the vehicle engine. A second variable pulley is mounted on an output shaft and is driven by the first pulley through the chain-belt. The second pulley acts through additional drive components to transmit torque to the vehicle drive wheels. Each pulley rotates about an independent shaft and is formed by the cooperation of two pulley sheaves, one of which is axially movable in a direction opposite from the other. The sheave pairs, mounted on the pulley axis, form the inner faces of the pulley. The profiles of the inner faces are generally inclined, such that the two sheave inner faces tend to converge toward the pulley axis. When at least one of the sheaves is movable axially relative to the other sheave, variation in the distance separating the opposing inner faces can be obtained.
The chain-belt which mechanically links the pulleys is generally a conventional chain-belt having a plurality of interconnected chain blocks, and may have a variety of link and block configurations, e.g. pin or rocker chains, link belts, etc. . . . The contact surface of the chain blocks engages the inner faces of the pulley sheaves and transmits torque by friction. Most conventional chain block configurations include chain blocks having contact surfaces which is a plane surface or very lightly crowned contact surfaces. In such configurations, the contacts with conventional conical sheaves are line contacts (i.e., where the contacted area is a band) and not point contacts. As the chain belt passes over the pulley, the point where the chain block or link articulates is defined as the pitch-line. The pitch (p) is the distance between successive articulations.
During the operation of the CVT, a movable sheave on the first pulley may be translated axially along the pulley axis so as to increase or decrease the distance separating the sheave inner faces. Similarly, a movable sheave on the second pulley decreases or increases the distance separating the sheave inner faces. Accordingly, as the distance separating the primary pulley inner faces is increased, the distance separating the secondary pulley inner faces is caused to be decreased in order to provide mechanical and hydraulic balance.
As the sheave members are translated along the axis of the pulley, the effective pulley radius is increased or decreased due to the inclined inner face of the sheave. The location of the chain-belt articulation or neutral axis (i.e., the pitch-line) around the intermediate circumference of the pulley inner faces defines the effective radius of the pulley. As the sheave inner face separation distance of the first pulley decreases, the chain-belt is forced to adopt its contact at a larger radial distance as it rises up along the inclined sheave inner face and the pitch-line is changed. Simultaneously, the effective radius of the second pulley is proportionately decreased by the separation of the pulley sheaves therein. Similar to the first pulley, the chain-belt is forced to adopt its contact at a smaller radial distance and the pitch-line is changed. Thus, the ratio of the pulley radii may be varied continuously to obtain the desired final drive ratio for the specific vehicle operating conditions.
In existing pulley sheaves, the inclined inner sheave faces are generally linear (conical). However, curved profile inner sheave faces are desirable to reduce the overall profile of the CVT pulleys. For CVT pulleys, the chain-belt centerlines on the drive and driven pulley translate along the axis of the pulleys from an initial centerline position. The difference between these translations causes an offset, while the chain-belt portion in each pulley remains in a plane perpendicular to the axes of the pulleys. Consequently, the chain-belt centerlines at the span between the pulleys tilt, or become inclined, relative to the centerline planes of the pulleys. This tilt or chain-belt misalignment, can cause uneven load distribution on the chain-belt, with associated wear and fatigue effects, and contribute to undesirable noise generation.